Discovery of Ancient Globular Cluster Candidates in The Relic, a Quiescent Galaxy at z=2.5

Abstract

Globular clusters (GCs) are some of the oldest bound structures in the Universe, holding clues to the earliest epochs of star formation and galaxy assembly. However, accurate age measurements of ancient clusters are challenging due to the age-metallicity degeneracy. Here, we report the discovery of 36 compact stellar systems within the 'Relic', a massive, quiescent galaxy at $z=2.53$. The Relic resides in an overdensity behind the Abell~2744 cluster, with a prominent tidal tail extending towards two low-mass companions. Using deep data from the UNCOVER/MegaScience JWST Surveys, we find that clusters formed in age intervals ranging from 8~Myr up to $\sim2$~Gyr, suggesting a rich formation history starting at $z\sim10$. While the cluster-based star formation history is broadly consistent with the high past star formation rates derived from the diffuse host galaxy light, one potential discrepancy is a tentative $\sim2-3\times$ higher rate in the cluster population for the past Gyr. Taken together with the spatial distribution and low inferred metallicities of these young-to-intermediate age clusters, we may be seeing direct evidence for the accretion of star clusters in addition to their early in-situ formation. The cluster masses are high, $\sim10^6-10^7~M_{\odot}$, which may explain why we are able to detect them around this likely post-merger galaxy. Overall, the Relic clusters are consistent with being precursors of the most massive present-day GCs. This unique laboratory enables the first connection between long-lived, high-redshift clusters and local stellar populations, offering insights into the early stages of GC evolution and the broader processes of galaxy assembly.

Figures (8)

• Figure 1: Left: Composite color image (F277W, F356W, and F444W) of the Relic (a) and two lower mass neighboring galaxies to the bottom right (b and c) connected by a diffuse tidal tail that extends roughly 100 kpc (40 kpc in the source plane). Right: The spectral energy distributions of the three galaxies are all consistent with being quiescent. The photometric redshifts for all three are consistent within the uncertainties. The scale bar is in the observed plane, whereas the source plane scale is roughly the same perpendicular to the main axis of the Relic and a factor of 2.5 smaller parallel to the main axis.
• Figure 2: Composite color image (F277W, F356W, and F444W) of the Relic before (left) and after (right) subtracting the main host galaxy. All point sources with a photometric redshift consistent with the main target ($z_{\mathrm{phot}}=2.53$) and SNR$>$3 in F444W are encircled, shown as white if the best-fit simple stellar population has a reduced $\chi^{2}<2$ and grey where reduced $\chi^{2}>2$. ID30 and ID32 (mauve) are removed from the final sample due tentative evidence suggesting extended sizes (see Section \ref{['sec:sizes']}). ID3, ID9, ID15, ID18, ID24, and ID31 (teal) only meet the SNR requires with an alternative photometry approach and are therefore not included in the final sample. The scale bar is in the observed plane, whereas the source plane scale is roughly the same perpendicular to the main axis of the Relic and a factor of 2.5 smaller parallel to the main axis.
• Figure 3: Measurements of the effective radius for cluster candidates in the five broadband filters redward of the Balmer break at $z=2.5$ reveal that all sources with robust fits (roughly 50% of the sample) are consistent with unresolved within the 16th to 84th percentiles of the posterior distribution. Given the faint magnitudes of the candidates, we assume a limit of 1.5 pixels (0.06$^{\prime\prime}$) to define this threshold. Measurements are color-coded by the filter they represent. ID30 (square symbols) is removed from subsequent analyses due to the combined brightness and tentatively extended size. ID32 (triangle symbols) is removed due to the extended size.
• Figure 4: Pseudo B-V versus V-I rest-frame color-color diagram through the F150W, F200W, and F277W filters for all compact sources detected near the Relic and the two neighboring galaxies along the tidal tail (see Figure \ref{['fig:neighbors']}) The symbols are color-coded by the age of their best-fit simple stellar population model, where larger symbols represent fits with reduced $\chi^{2}<2$ and smaller circles are for reduced $\chi^{2}>2$. The color of the main Relic is indicated with a star and diffuse light in the tidal tail with a square symbol. The arrow represents the typical dust attenuation for ages less than 10 Myr of 0.2 mag, assuming the median dust index value of -0.9. FSPS model tracks for 25% Solar metallicity at $z=2.53$ are shown (solid line), with Solar metallicity indicated with a thinner higher transparency line. The 25% Solar metallicity FSPS model track for the same three filters but now at $z=0.308$, the redshift of the Abell 2744 cluster, is shown with a dotted line.
• Figure 5: Example SEDs of the 20 stellar clusters with reduced $\chi^{2}$ of the best-fit simple stellar population is less than 1.9 at $z=2.53$. The color image (top right) shows the full sample of 36 sources, with the apertures color-coded by the maximum a posteriori (MAP) age from the single burst modeling analysis. We additionally show the extracted SED and best-fit model for three regions tracing the diffuse light of tidal features (D1, D2, and D3); the box regions on the image are similarly color-coded by the best-fit MAP age. The scale bar is in the observed plane, whereas the source plane scale is roughly the same perpendicular to the main axis of the Relic and a factor of 2 smaller parallel to the main axis.